Added errorprone check to flag context#getApplicationContext calls.

The overall goal is to prevent storing the result of calls to
context#getApplicationContext. We previously removed them all, but more have
snuck in since.

Bug: 846456
Change-Id: I6ff008b75a3980cc2db770ea42a18b9ee3669cae
Reviewed-on: https://chromium-review.googlesource.com/1082666Reviewed-by: Peter Wen <wnwen@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577154}

tools/android/errorprone_plugin/BUILD.gn

@@ -6,6 +6,8 @@ import("//build/config/android/rules.gni")
 
 java_library("errorprone_plugin_java") {
   java_files = [
+    # Turned off because of existing code which fails the check
+    # "src/org/chromium/tools/errorprone/plugin/NoContextGetApplicationContext.java",
     "src/org/chromium/tools/errorprone/plugin/NoSynchronizedMethodCheck.java",
     "src/org/chromium/tools/errorprone/plugin/NoSynchronizedThisCheck.java",
   ]

tools/android/errorprone_plugin/src/org/chromium/tools/errorprone/plugin/NoContextGetApplicationContext.java

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.tools.errorprone.plugin;
+
+import static com.google.errorprone.matchers.Matchers.instanceMethod;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.suppliers.Supplier;
+import com.google.errorprone.suppliers.Suppliers;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.tools.javac.code.Type;
+
+/**
+ * Checks for calls to getApplicationContext from {@link android.content.Context}.
+ * These calls should be replaced with the static getApplicationContext method in
+ * {@link org.chromium.base.ContextUtils}.
+ */
+@AutoService(BugChecker.class)
+@BugPattern(name = "NoContextGetApplicationContext",
+        summary = "Do not use Context#getApplicationContext",
+        severity = BugPattern.SeverityLevel.ERROR, linkType = BugPattern.LinkType.CUSTOM,
+        link = "https://bugs.chromium.org/p/chromium/issues/detail?id=560466")
+public class NoContextGetApplicationContext
+        extends BugChecker implements BugChecker.MethodInvocationTreeMatcher {
+    private static final String CONTEXT_CLASS_NAME = "android.content.Context";
+    private static final String CONTEXT_UTILS_CLASS_NAME = "org.chromium.base.ContextUtils";
+    private static final String METHOD_NAME = "getApplicationContext";
+
+    private static final Supplier<Type> CONTEXT_UTILS_SUPPLIER =
+            Suppliers.typeFromString(CONTEXT_UTILS_CLASS_NAME);
+    private static final Matcher<ExpressionTree> CONTEXT_MATCHER =
+            instanceMethod()
+                    .onDescendantOf(Suppliers.typeFromString(CONTEXT_CLASS_NAME))
+                    .named(METHOD_NAME);
+
+    @Override
+    public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState visitorState) {
+        if (!CONTEXT_MATCHER.matches(tree, visitorState)) {
+            return Description.NO_MATCH;
+        }
+
+        // If ContextUtils can't be loaded, we are probably inside a third_party lib and shouldn't
+        // check for errors.
+        boolean canLoadContextUtils = CONTEXT_UTILS_SUPPLIER.get(visitorState) != null;
+        if (!canLoadContextUtils) {
+            return Description.NO_MATCH;
+        }
+
+        return buildDescription(tree)
+                .setMessage("Don't use Context#getApplicationContext - "
+                        + "call ContextUtils.getApplicationContext instead")
+                .build();
+    }
+}